1. Field of the Invention
This invention relates to a mass flow control method and device utilizing a sonic nozzle.
2. Related Art Description
In recent years as a variety of industries have developed, there has been an increasing need for controlling mass flow rate, and in many fields manufacturing and the like cannot be done properly unless mass flow rate can be controlled accurately. For example, in the field of semiconductor industry, as the fine processing technique developed, high integration also progressed, and as a semiconductor manufacturing process becomes subdivided, the control of a thin film developed on a wafer is becoming more important. It is possible to control the thin film growth by controlling trace amounts of process gas flow. Accordingly, in order to control the flow rate of fluids such as process gas and the like used in such a manufacturing process, not only reproducibility and stability but also mass flow rate as a precise absolute value is required. At present, the flow of fluids such as process gas and the like is mostly controlled by means of a thermal mass flow meter. However, if a conventional mass flow meter is used to control mass rate of flow, in some cases trace amounts of flow cannot be expected to be controlled accurately and correctly at a due level.
A sonic nozzle is employed for a mass flow meter. Generally, the sonic nozzle is used in the medium flow range of about 50/min. or more in mass flow rate. The sonic nozzle has a fixed discharge coefficient and the degree of variability thereof is 0.1% or less in the area where the Reynolds number is 10.sup.6 or more. By forming a sonic flow with the relation between an upstream fluid pressure and a downstream fluid pressure kept less than the critical pressure ratio, it is possible to supply a fluid at a fixed mass flow rate irrespective of fluid conditions such as the downstream fluid pressure, temperature, and the like, whereby a fixed mass flow rate can be set in a manner extremely accurately. In the area where the Reynolds number is smaller than 10.sup.6, however, the discharge coefficient greatly varies non-linearly; and in the above-mentioned area, namely, in the area where a flow rate is extremely low, no sonic nozzle is utilized.